Many times you will be asked this question in your interviews. e.g. How Iterator
works in java, how it remove objects from List
? While removing objects from list, when it may produce ConcurrentModificationException
?
In Java, when you create an iterator, it starts to count the modifications that were applied on the collection. If the iterator detects that some modifications were made without using its method (or using another iterator on the same collection), it cannot guarantee anymore that it will not pass twice on the same element or skip one, so it throws this exception.
ArrayList
methods always increase a counter ‘modCount‘ declared as:
protected transient int modCount = 0;
When you create an Iterator
/ListIterator
on arraylist
, this modCount is used to initialize expectedModCount
for that Iterator
instance.
int expectedModCount = modCount;
Any operation done on ArrayList directly (without using iterator instance) will increase the modCount. Iterator
’s all methods perform a check to validate the equality of modCount == expectedModCount
– before performing the operation. If they do not match, it simply throws the exception.
final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); }
I hope above explanation will help you answer this interview question.
Happy Learning !!
Anil
Hi Lokesh,
“ArrayList methods always increase/decrease a counter ‘modCount‘ declared as:”
I think it should be only increase, else please clarify.
Thanks
Anil
Lokesh Gupta
You are right.
Nishant Kumar
awesome !!!